So, yeah: It’s vitamin C’s world and we’re all just living in it. But, as it happens, the ingredient also might have a kryptonite that keeps it (and your complexion) from reaching its full potential.

According to Joshua Zeichner, MD, a New York City-based dermatologist, vitamin C is pretty hard to keep on lock. “Think about vitamin C like a charm bracelet. The bracelet itself is stable around your wrist, but the charms are very delicate and easily broken,” he explains. “You can wear your charm bracelet, but need to take extra special care not to bang it around too much when it is on your wrist.”

Similarly, vitamin C is stable on its own, but Dr. Zeichner says that it’s sensitive to light and requires a very specific acidic pH to remain stable. Beyond just that, it can also easily become oxidized when it’s exposed to air, meaning that you can kiss that litany of complexion-boosting benefits buh-bye.

So, how do you know when it’s time to say “See ya later!” to your vitamin C serum? Turns out, it’s pretty easy. “If your serum started out clear and now looks amber colored, it is time for a new bottle,” says Dr. Zeichner, noting that this yellow coloring indicates that it’s begun to oxidize. And if it feels or smells different than it did when you bought it (AKA if it starts to get goopy or takes on a fresh scent), it’s time to spring for a new bottle of this liquid gold.